Wood can be used to construct almost any part of a home from the roofing and exterior walls to the floor and interior architectural elements as well as basic domestic items like furniture and cabinets. However, in recent years the cost of solid timber wood has increased dramatically as its supply shrinks due to the gradual depletion of old-growth and virgin forests.
Accordingly, because of both the cost of high-grade timber wood as well as a heightened emphasis on conserving natural resources, wood-based composite materials alternatives to natural solid wood lumber have been developed that make more efficient use of harvested wood and reduce the amount of wood discarded as scrap. Plywood, oriented strand board (“OSB”), laminated veneer lumber (LVL), parallel strand lumber (PSL), and laminated strand lumber (LSL), and oriented strand lumber (OSL), are examples of wood-based composite alternatives to natural solid wood lumber that have replaced natural solid wood lumber in many structural applications in the last seventy-five years.
Pressed boards and wood composite materials are manufactured by mixing wood and one or more additives, such as adhesives and waxes. During manufacture, the wood-additive mixture is first laid down in batches on a conveyor belt in a loose mat, and this loose mat is then simultaneously compressed and heated. Heating the mat cures the binder and waxes present in the wood-additive mixture as well as evaporates the moisture present in the raw materials, while simultaneously, by the action of compression, the wood and additive materials are fused together to form a consolidated wood board.
Compression of the wood and wood-additives into a wood composite material may occur in either a multi-platen press where several batches of wood and wood additives are set upon a series of press platens, and the batches compressed between adjoining platens, or in a continuous process, where a wood composite material is made by continuously moving a wood and wood additive mat between two heated steel belts that apply pressure to the mat to from a billet or sheet of wood composite material that is then cut into a predetermined length to form boards of a manageable size.
It has been previously noted that in the compression of the wood composite material by either of the aforementioned methods, that preheating the mat of wood and wood additive material with steam can dramatically reduce the press time, which is the amount of time necessary for the adhesive to set or “cure” within the wood composite board to give the board its coherency and strength and to consolidate the material into a wood composite board. U.S. Pat. No. 5,733,396 discloses preheating a particle or wood strand mat to a temperature of less than 100° C. with a mixture of super-heated steam and hot air. The steam/hot air mixture provides moisture to soften the wood fibers and enhance lignin flow in the mat. Preheating with steam/hot air is especially effective with polymeric resin such as isocyanate adhesives and resins because isocyanates readily react with water, hydroxyl, and other functional groups found in lingo-cellulosic materials. Further advantages include reduction in the amount of volatile organic compound (“VOC”) emissions because of the mild press parameters (e.g., pressure, exposure time and temperature).
Unfortunately, several problems concurrent with the usage of steam preheating step have been observed, especially for panels having a final pressed thickness of greater than 1 inch. The most common of these problems in wood composite panels include blistering, carbonizing, surface pitting, delaminating, and warping. All of these aforementioned imperfections can all be traced to aspects of the preheating process. For example, blistering on both the panel surface and interior occurs as a result of non-uniform moisture condensation, incomplete steam penetration, and a sudden reaction between an isocyanate resin (particularly “MDI” which is discussed in greater detail below) and water vapor which are due all or in part to the steam preheating step. Surface pitting is similarly caused by a steam preheating step, as a result of the impact of the steam flow injected at high pressure towards the panel. Additionally, other defects such as a large degree of warping and thickness swelling have been noticed, especially in wood strand lumber products with uni-directional laminated strands.
Other processing strategies, modifications or requirements have been developed to avoid the aforementioned imperfections. For example, in order to avoid blistering and carbonization, it is necessary to use lower press temperatures that require longer pressing cycles to ensure proper composite consolidation. Use of steam preheating can create a need for a prolonged de-gassing step in order to obtain products that meet the performance requirements and avoid blows and delaminations, especially for wood strand lumber products using long strands. Furnish moisture content has to be tightly controlled in the manufacturing process with a narrow tolerance. Often, special manufacturing adjustments/change have to be made. In order to implement such manufacturing changes, it is necessary to install special processing technology and equipment changes to simultaneously reduce the press cycle time while also maintaining the high quality of the wood composite materials. Such process adjustments not only increase production costs, but also reduce the quantity and quality of wood composite materials that can be manufactured.
Given the foregoing there is a continuing need for an apparatus and method for producing composite wood products whereby the benefits of steam preheating may be obtained without reducing the throughput, and undermining the quality of the wood composite materials that are manufactured.